1. Field of the Invention
The present invention relates to a tunable filter to be used when selecting from, eliminating out of, or adding to a wavelength division multiplex optical signal a particular wavelength, mainly in an optical communication network in which wavelength division multiplexing is employed, and manufacturing method thereof, and further to an optical switching device comprising such tunable filter.
2. Description of the Related Art
Recently, wavelength division multiplexing has come to be popularly used in a field of optical communication, making it more and more important to select and pick up a particular wavelength out of a wavelength division multiplex (hereinafter referred to as “WDM”) optical signal. An optical add/drop device and an optical cross-connect device, which are currently available in the market, are generally designed to select a fixed wavelength out of a WDM optical signal. In such fixed wavelength filters, generally a Fabry-Perot etalon filter comprising a dielectric multi-layer film is incorporated because of its excellent cost performance.
Also, the U.S. Pat. No. 4,825,262 discloses a variable wavelength type Fabry-Perot etalon filter that can vary a wavelength by controlling a voltage. Further, the U.S. Pat. No. 5,739,945 discloses a Fabry-Perot etalon filter having a semi conductor/air constitution based on MEMS (Micro Electro-Mechanical System) technology, for increasing a resolution for selecting a wavelength.
Further, Japanese Patent Laid Open Publication (JP-A) H11-142752 discloses an interference filter of a variable transmitted wavelength type. In this variable transmitted wavelength type interference filter, a pair of substrates is disposed in parallel and a multi-layer film is formed on the respective confronting surfaces of the substrates with a certain clearance therebetween, which clearance is variable by an external force.
FIG. 11 shows a conventional tunable filter similar to the variable wavelength interference filter set forth in the foregoing literature. An optical multilayer film 1003 is formed on a substrate 1001, which is a silicon substrate. The optical multilayer film 1003 is composed of films of a high refractive index and films of a low refractive index, alternately layered. The high index film consists of a Ta2O5 film while the low index film consists of an SiO2 film, both of which are layered by ion beam deposition. Further an Au film is formed as an electrode 1004a on the optical multilayer film 1003 by sputtering.
In a manufacturing method of such tunable filter, a polyimide film (not shown) is first deposited to form a sacrificial layer, so that a cavity gap 1006 is formed by sacrificial layer etching. An Au film is then formed as an electrode 1004b on the polyimide film by sputtering. Then an Si3N4 film is formed by sputtering, which serves as a spring portion 1005. Further, another optical multilayer film 1002 is formed in a similar way to the optical multilayer film 1003, i.e. by forming alternate layers of Ta2O5 films used as high index film and SiO2 films used as low index film utilizing ion beam deposition method. Also, a portion on the substrate 1001 where a transmitted light beam 1009 is to go through is selectively eliminated by silicon crystal anisotropic etching utilizing KOH. And then the sacrificial layer is removed by oxygen plasma ashing, so as to form the cavity gap 1006.
In a tunable filter as constituted above, a clearance between the optical multilayer film 1002 and the optical multilayer film 1003 (cavity gap 1006) can be electrostatically varied by applying a voltage between the electrodes 1004a and 1004b. By controlling the cavity gap 1006 in this way, a wavelength that matches the cavity gap out of the WDM light 1007 can be selectively transmitted, so that a transmitted light beam 1009 is obtained. The remaining portion of the WDM light is reflected and turns into reflected light 1008. Consequently, only light of a desired wavelength passes through the filter, and the wavelength to be transmitted can be varied by controlling the cavity gap 1006.
As a result of performance measurement of a tunable filter of the conventional constitution as above, bandwidth for −1 dB was 0.20 nm, adjacent channel cross talk value was −10.5 dB, tuning range was 1520 to 1630 nm, and insertion loss was 2.1 dB.